List value problem, python
there is a rookie problem. The code is as follows:
arr = [1,2,3,4,5,6,7,8,9,10]
print(arr[1:5])
print(arr[1:5:2])
print(arr[1:5:-2])
:
[2, 3, 4, 5]
[2, 4]
[]
- does the second parameter 5 also represent the index, and if so, is it intercepted before the index? Can you understand it that way?
- you can take a value when the step size is 2. Why is it empty when-2
- what is the underlying principle of intercepting the value of the index here in the
- python list, and why it leads to the above result
< hr >
< hr >
Thank you. I understand. The code can see the answers that have been adopted.
if it is easy to understand, it is the explanation of dodopy . Post it down:
:
1.0
2.
3.
4.step1
arr = [1,2,3,4,5,6,7,8,9,10]
-sharp 2index141[2,3,4,5]
print(arr[1:5])
-sharp 2index142[2,4]
print(arr[1:5:2])
-sharp 2index142[]
print(arr[1:5:-2])
-sharp
-sharp 142[10, 8]
print(arr[-1:-5:-2]
-sharp
def index(l, start=None, end=None, step=1):
result = []
if step == 0:
-sharp raise
return
elif step > 0:
if start is None: start = 0
if end is None: end = len(l)
while start < end:
result.append(l[start])
start += step
else:
if start is None: start = len(l) - 1
if end is None: end = -1
while start > end:
result.append(l[start])
start += step
return result
l = list(range(10))
print(l[::2], index(l, step=2))
print(l[::-2], index(l, step=-2))
print(l[:5:2], index(l, end=5, step=2))
print(l[:6:-3], index(l, end=6, step=-3))
print(l[6::2], index(l, start=6, step=2))
print(l[2::-2], index(l, start=2, step=-2))
print(l[2:7], index(l, start=2, end=7))
print(l[7:2:-1], index(l, start=7, end=2, step=-1))
of course, the bottom layer of list slices is implemented in c, which is much more efficient than what I wrote
clarify several messages:
1. The index of the list starts at 0
2. The range of slices conforms to the principle of closing left and opening right
3. A negative number indicates the order from right to left
4.step value defaults to 1
arr = [1,2,3,4,5,6,7,8,9,10]
-sharp 2index141[2,3,4,5]
print(arr[1:5])
-sharp 2index142[2,4]
print(arr[1:5:2])
-sharp 2index142[]
print(arr[1:5:-2])
-sharp
-sharp 142[10, 8]
print(arr[-1:-5:-2]
When
step is negative, start is greater than end,
everything else is correct.
https://stackoverflow.com/que.
this is the operation of list slices
aa= [1 br > aa [1:5:2] refers to slices with an interval of 2.
AA [1: 5] refers to slices with an interval of-2. Negative numbers can only be used when slicing in reverse, so the result is empty.
attach a picture and knock on it for yourself.
